These joints incorporating elastomeric rings have been in common use on pipeline systems for many years. They provide a cost effective method of joining pipes and have been generally proven to be very reliable in service. By themselves, however, they do not provide end restraint, which is to say that they will not prevent the joints between pipes from becoming separated if the lie of the pipeline could lead to this possibility. This situation is possible where the pipeline direction changes and no other steps are taken to account for the pressure thrust so generated.
With previous joints attempts to weld plastic coated and lined pipes, and thereby to provide the necessary restraint, has resulted in the heat being conducted through the steel on the spigot causing damage to the internal lining. Additionally with these joints heat was conducted along the lip of the socket and into the area where the elastomeric ring resides. This heat then softens the internal coating of the socket which causes the elastomeric material of the rings to expand radially and the compression force of the ring between the internal surface of the socket and the external surface of the spigot diminishes. This loss of compression enables water to leak past the elastomeric sealing ring and into the welded joint area where corrosion can take place.
Another method of joining pipes that does provide full restraint is an externally welded lap weld. This method is only applicable when the lining material is cement mortar. The inherent flaw in this method is that there is always an exposed steel section inside the pipe that can corrode when exposed to certain water chemistry. This method is also not applicable when the lining is of a plastic or other similar type coating.
The present invention provides a system that allows the joint to be formed and deflected to the required angle, and welded while still maintaining the continuous internal corrosion protection coating.